Send via office common email?

I'm new to all the Outlook setup stuff as I am a C# programmer so bare with my terminology.

The office has a common email 'sales@domain.com' of which I am required to have anybody in the office to be able to send emails out via this email address.

This so far has already been set up on one computer before my employment and emails are sent via the 'sales@domain.com' by selecting the 'From' button.

So far the closest I've got is sending out emails in this manner has resulted with 'on behalf off' this email address, but this is not sufficient enough.

As far as I know, all computers are using outlook 2003 and the exchange server is also 2003.

My attempts so far have consisted of accessing 'Active Directory Users and Computers' -> '[My Business].local' -> 'MyBusiness' -> 'Users' -> 'SBSUsers', then selecting the properties of the email account -> 'Exchange Advanced' tab -> 'Mailbox rights' and playing around in there with no luck.